Finally, to determine if software errors could explain the reported effect, images were acquired in the model eye as well as human subjects using the Spectralis, and optical power-related eye data were varied before each scan, including axial length, corneal curvature, and refraction. This study followed the tenets of the Declaration of Helsinki and was approved by the institutional review board at the Medical College of Wisconsin. Axial length categorizations according to Heidelberg (short, medium, long), as well as axial lengths of the three subjects measured with an IOL Master (Carl Zeiss Meditec, Dublin, CA, USA) are given in the
Table. Three subjects were imaged on the Spectralis and three masked observers used the built-in calipers to measure the distance between the foveal surface and the retinal pigmented epithelium (RPE) in three separately acquired B-scans. Observers chose the B-scan with the lowest foveal depression, and made measurements on the unsmoothed image at the highest digital zoom available to provide the greatest precision. The order of the images analyzed was randomized separately for each observer. Prior to acquisition, a new patient ID was created, and either the axial length setting, corneal curvature, or refraction was varied (
Figs. 4C–E). The effect of axial length setting on apparent thickness was assessed by
n-way ANOVA to account for interobserver and intersubject variability, and no significant effect was found (
Fig. 4C,
P = 0.27). The effect of both corneal curvature setting and refraction setting on apparent thickness was assessed by 2-way ANOVA, and no significant effect was found (
Fig. 4D, corneal curvature:
P = 0.23;
Fig. 4E, refraction:
P = 1.00). These experiments were repeated in the model eye to eliminate possible confounds due to imperfect alignment, and similar results were obtained (
Figs. 4F–H). The results obtained by varying the axial length setting are compared to the reported model for reference
1 (
Fig. 4F). The effect of corneal curvature on apparent thickness was assessed by Pearson's correlation and linear regression (
Fig. 4G;
P = 0.56,
R2 = 0.13), and the difference in apparent thickness between a −4 and +10 diopter refraction setting was only 1 μm (
Fig. 4H).